Soft switching is a technique used in power converters. Soft switching can implemented in all the DC-DC converters such as buck, boost and bridge converters. Nevertheless, achieving complete soft switching for full bridge and push pull converters, wherein two or more active switches are already present, is relatively easy compared to boost converters. Soft switching improves system efficiency, allows for high frequency of operation and generates less EMI (Electromagnetic Interference).
Generally, soft switching is achieved either with the help of active snubbers, or by incorporating an auxiliary switch or with resonant switching.
FIG. 1 depicts a conventional boost regulator. Lb is the boost inductor, SW1 is the main switch, Db is the boost diode and Co the output capacitor. Energy is stored in the boost inductor when SW1 is ON for Ton duration and the same energy is delivered to the load adding up to input voltage when SW1 goes OFF for Toff duration. Output voltage V0 is regulated by varying the duty cycle of SW1 and V0 is always higher than the input voltage Vin. The input and output voltages are related byV0=Vin/(1−D)where D is the duty cycle defined asTon/(Ton+Toff)  (1)
Though Continuous Conduction Mode (CCM) control of the boost converter provides lower input peak currents and associated lower losses, it also has some disadvantages such as reverse recovery problem of the boost diode and also Right Half plane Zero stability issue. Especially at high frequencies and higher output voltages, the reverse recovery of the boost diode causes enormous stress on the boost switch and the losses in the switch can turn out to be detrimental.
A Boundary Conduction Mode (BCM) boost converter does not have the reverse recovery issue with boost diode and also the Right Half Plane issue. This mode is widely adapted for the lower power levels up to say 200 to 300 Watts. Since the inductor (Lb) current starts from zero value in each switching cycle, the peak currents are much larger, and in some cases, it can be as high as 6 times the average current. Therefore for higher power levels in the region of 500 watts and above, BCM is not tenable.
In CCM boost converter, the boost switch turns ON and OFF with reasonable currents, depending on the output power level. Drain Voltage swing also will be from zero to the output voltage.
The switching losses in the boost switch can be computed asPsw=Iinmax*V0*(ton+toff)/T*2  (2)wherein Iinmax is the input current through the switch at the end of ON time, T is the time period of the switching cycle, ton and toff are the rise and fall times of the switch.
The switching power loss in the switch for a 1.0 kW boost converter, operating with 220V DC input, and 400 VDC output and switching around 200 kHz, would be around 25 Watts. Such a large dissipation in a single semiconductor device poses a big challenge in the practical applications.
Another contributor is the power loss in the boost switch due to reverse recovery of the boost diode. When the boost diode is turned OFF, it does not block the current to zero instantaneously, and thus acts like a short circuit for the reverse recovery time. The entire output voltage is impressed across the boost switch, which is in the ON state. For the duration of reverse recovery of the boost diode, the boost switch is stressed enormously. The loss induced thus, is regenerative in nature and may cause the thermal runaway of the boost switch at high frequencies. Further, the loss induced by the energy stored in the output capacitance Coss of the boost switch at 400V DC is added to this. Assuming a capacitance of about 200 Pf, the loss due to this can be as much as 3.0 Watts. Therefore nullifying the switching losses (soft switching) improves the system efficiency and also the reliability.
It can be seen that most of the current schemes have extra active switches and a higher number of power components to achieve soft switching.